Polyamide-imide resin is high in heat resistance, chemical resistance, wear resistant, etc., and has been used in producing industrial machine parts, films, electric/electronic parts, automobile parts, and aerospace related components. When polyamide-imide is used for a coating, it is commonly dissolved in an organic solvent and applied as varnish. In recent years, however, reduction in volatility organic compounds (VOCs) has become an important issue and there is a strong call to minimize VOC consumption. A solution is to use polyamide-imide in the form of fine powder. Fine powder can be directly sprayed on a component, or coating can be conducted by preparing an aqueous dispersion and spreading it on a component followed by removing water. Furthermore, fine particles of polyamide-imide resin may be added to other resins or composite materials to develop thixotropy or enhance impact resistance, or may be mixed with other resins to provide materials for shims of disk brakes or the like.
In this way, fine particles of polyamide-imide resin have been used in many application fields and expected to find many new uses in the future. However, they should be dispersed uniformly in water or resin if good properties of polyamide-imide resin are to be drawn on by mixing it with water or resin. To this end, it is preferable that these fine particles have an average primary particle diameter of 300 nm or less, and it is particularly preferable that they have a uniform diameter of 200 nm or less.
A known method to synthesize fine particles of polyamide-imide resin is subjecting a first solution containing an acid chloride such as trimellitic anhydride chloride and a second solution containing a diamine compound such as 4,4′-diaminodiphenyl ether to ultrasonic agitation in the presence of a solvent soluble in both the first and second solutions to produce fine particles of polyamide-imide resin (Japanese Patent No. 4304434, and Japanese Unexamined Patent Publication (Kokai) Nos. 2005-97370 and 2006-257345). This method, however, requires a special apparatus to generate ultrasonic waves and has many disadvantages in producing fine polyamide-imide particles industrially. These publications, furthermore, only address, as a specific example, production of fine polyamide particles from dicarboxylic acid chloride, used as acid chloride, and disclose no examples of producing fine particles of polyamide-imide resin.
Elsewhere, a method for producing fine particles of polyamide-imide resin by spray drying has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. HEI-4-285660. In that method, polyamide-imide resin dissolved in N-methyl-2-pyrrolidinone is spray-dried using a Mobile Minor type spray drier to produce fine polyamide-imide particles with an average particle diameter of 4.5 μm. Some studies have disclosed methods for producing, for instance, fine particles of polyamide-imide resin with an average particle diameter of 3 μm through polymerization of 4,4′-diphenylmethane diisocyanate and trimellitic anhydride (Japanese Unexamined Patent Publication (Kokai) Nos. HEI-11-246759 and 2000-17073). It is impossible for these methods, however, to produce fine particles of polyamide-imide resin of 1 μm or less.
In another disclosed method, polyamide-imide resin dissolved in 1,3-dimethyl-2-imidazolidinone is added to an aqueous solution of a surface active agent having a phenyl group, followed by precipitating fine particles of the polyamide-imide resin (Japanese Unexamined Patent Publication (Kokai) No. 2009-067880). Although able to produce fine particles of polyamide-imide resin of 1 μM or less, this method is low in reproducibility because the particle diameter varies largely depending on the concentration of the surface active agent, the rotating speed of the stirrer, and the time elapsing before the start of dripping of the solution of polyamide-imide resin into the aqueous solution of a surface active agent. As a result, it is not always possible to obtain fine particles of polyamide-imide resin of 1 μm or less. In addition, that method is limited to 1,3-dimethyl-2-imidazolidinone as the solvent to dissolve the PAI resin and requires high speed stirring at 1,000 to 4,000 rpm, leading to various problems in developing industrial production processes.
Under such circumstances, there is now a strong call for development of a process that can perform mass production with a high reproducibility of highly uniform fine particles of polyamide-imide resin with an average primary particle diameter of 300 nm or less, preferably 200 nm or less.
Thus, it could be helpful to provide a process to produce fine particles of polyamide-imide resin (hereinafter, “polyamide-imide” may be abbreviated as PAI) that can be carried out industrially by simple operations.